基于Mogi-Coulomb强度准则的井壁稳定性力学分析新方法*

传统井壁坍塌失稳分析模型是基于M-C破坏准则而建立的，该方法忽略了中间主应力对岩石强度的影响，常使得预测的钻井液密度过于保守。为此，引入考虑中间主应力的Mg-C破坏准则建立任意井眼的井壁稳定性评价新模型，通过数值模拟法分析了不同地应力状态下的井斜角和方位角对井壁稳定性的影响。研究结果表明: Mg-C准则克服了M-C准则忽略中间主应力低估岩石强度的影响，其计算结果略低于M-C准则的计算结果，有助于提高钻速，降低钻井成本；正断层地应力状态时，沿最小水平主应力方向钻进时，井壁稳定性最好；走滑断层地应力状态时，沿最大与最小水平主应力之间的某一临界角钻进时井壁最稳定；逆断层地应力状态时，沿最大水平主应力方向钻进时井壁最稳定；该模型有助于指导现场钻井井眼轨迹的优化设计及安全施工。

New method for mechanical analysis of wellbore stability based on Mogi-Coulomb strength criterion

The traditional analysis models of wellbore collapse instability have been established based on the M-C failure criterion, which neglected the influence of the intermediate principal stress on the rock strength and often made the predicted drilling fluid density too conservative. Therefore, the Mg-C failure criterion considering the intermediate principal stress was introduced to establish a new model of wellbore stability evaluation for any wellbore, and the influence of deviation angle and azimuth angle on the wellbore stability under different in situ stress patterns was analyzed by using the numerical simulation method. The results showed that the Mg-C criterion overcame the influence of the M-C criterion on ignoring the intermediate principal stress and underestimating the rock strength, and its calculation results were slightly lower than those of the M-C criterion, which contributed to increase the drilling rate and reduce the drilling cost. Under the normal fault in situ stress pattern, the wellbore stability was the best when drilling in the direction of the minimum horizontal principal stress. Under the strike slip fault in situ stress pattern, the wellbore stability was the best when drilling along a certain critical angle between the maximum and minimum horizontal principal stress. Under the reverse fault in situ stress pattern, the wellbore stability was the best when drilling in the direction of the maximum horizontal principal stress. This model is helpful to guide the optimization design and safe construction of the hole trajectory in the field well drilling.